In prior art, e.g. document JP 2005087876 discloses a hydrofoil impeller with three blades and comprising a central hub which is connected to a rotatable shaft. The central hub is in the form of a flat plate with a uniform thickness and being perpendicular to the central axis of the shaft. The central hub has three groups of four first bolt holes arranged to form a pattern. The number of groups of first bolt holes corresponds to the number of blades attached to the central hub. Thus, three blades extend radially outwardly from the central hub, each blade having a root portion. The root portion is in a form of a flat plate with a uniform thickness, said root portion has a group of four second bolt holes. The second bolt holes are arranged in a corresponding pattern in relation to the pattern of the first bolt holes so that the group of second bolt holes can be aligned with the group of first bolt holes and bolts can be placed through the first and second bolt holes to form bolted joints.
A problem with the attachment of blades to the central hub of the impeller with bolted joints wherein the bolts are arranged in linear rows is that the dynamic stresses caused by fluid forces, when the impeller is in operation during agitation of the fluid, are unevenly exerted at the bolted joints and the material of the hub and blades near to the bolted joints affecting strongly on the fatigue life of the impeller. Fatigue occurs when a material is subjected to repeated loading and unloading. If the loads are above a certain threshold, microscopic cracks will begin to form at the stress concentrators. Eventually a crack will reach a critical size, the crack will propagate suddenly, and the structure will fracture.